Can we make tailpipes that capture CO2?

Image Gallery: Environmental Issues Smog over Beijing, China, in May 2008. The nation is the largest emitter of carbon dioxide; the United States is a close second. See more pictures of environmental issues.

Around the world, people are growing increasingly concerned about carbon dioxide (CO2) emissions. Certainly, climate change skeptics pose reasonable hypotheses that suggest changes in climate are merely a natural, global cycle -- and we humans are just going to have to ride out. But the idea that humans are contributing to climate change is becoming more accepted. In response, scientists are thinking of ways to reduce humans' greenhouse gas (GHG) emissions.

One way is to create fuels that don't produce carbon dioxide as a byproduct, like fossil fuels do. Biofuels like cellulosic ethanol made from corn or switchgrass still emit CO2 when burned for energy, but in far smaller amounts -- as much as 85 percent less [source: Wang]. Burning hydrogen to power a car produces no carbon dioxide; the only byproduct is water. And electricity produced from renewable resources like wind or solar power doesn't produce any emissions at all.

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The problem with these technologies is that they're still being developed. Researchers are facing obstacles like cost and net energy ratio -- input of energy versus energy output -- that make oil more attractive than alternative fuel sources. This is significant, because our world is powered by oil. From the airplanes that make travel possible, to the trucks that transport food and the power plants that produce our electricity, oil dominates the global economy.

It's a pretty good question: If we're dependent on oil but concerned about carbon dioxide emissions, why don't we just capture the CO2 we emit?

Actually, researchers are looking into this right now. Professor Chris Jones at the Georgia Institute of Technology (Georgia Tech) and his team have come up with a material called hyperbranched aminosilica (HAS) that captures and stores carbon dioxide emissions.

So will we soon find tailpipes on cars made of HAS, and what exactly is this material anyway? Find out on the next page.

Hyperbranched Aminosilica

So will our cars' tailpipes be made of this stuff called hyperbranched aminosilica (HAS) in the near future? Dr. Chris Jones says he doesn't think so; storing captured carbon from all those tailpipes would be too costly. Instead, Jones and his team at the Georgia Institute of Technology (Georgia Tech) are focused on an even bigger source of carbon dioxide emissions -- power plants.

You may think of electricity as clean energy. But have you ever considered where electricity comes from? Since it's an energy carrier, electricity gets its energy from another source. In the United States the majority of that energy -- 50 percent -- comes from coal [source: Pew]. Electrical power plants worldwide use enough fossil fuels for energy production to account for 26 percent of global CO2 emissions; transportation (including planes, trains and automobiles) account for 13 percent worldwide [source: IPCC].

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Jones has his sights set on cleaning up smokestacks. HAS can help by adsorbing CO2. The Georgia Tech researchers used covalent bonding (combining two molecules by joining their electrons) to bind amines -- nitrogen-based organic compounds -- with silica (quartz) [source: Georgia Tech]. The result is aminosilica, a powdery substance that looks like white sand. Within the substance, a number of branches that resemble trees are born from the bonding, hence the name: hyperbranched. At the braches' tips are amino sites that capture CO2.

When HAS was combined with sand, the chemists found that the resulting compound was capable of trapping carbon dioxide when flue gasses -- emissions found in smokestacks -- passed through it.

The HAS compound not only captures CO2, it hangs onto it. To release the carbon dioxide, the material must be heated, and the CO2 that's released can be captured and stored (either as a gas or cooled into liquid form) in a process called carbon sequestration. This is actually more exciting than it sounds. Not only will it reduce CO2 emissions, it makes it possible to reuse the captured CO2 to feed biofuel stock. One company grows algae in Louisiana for use as a biofuel. The algae are fed with captured CO2 [source: EcoGeek].

Hyperbranched aminosilica has some advantages over other methods of carbon sequestration. For one, it's recyclable. HAS can be used over and over again; the Georgia Tech researchers tested one batch 12 times and found that there was no noticeable decrease in adsorption [source: Georgia Tech]. And the material also isn't affected by moisture, which is a plus since water vapor is present in flue gases. It's also low on required energy input; the only energy needed comes from the generation of the heat that releases the CO2.

But there are some challenges that face the project. For one, the CO2/amine reaction that binds the carbon dioxide to the branches generates heat. The researchers found that the aminosilica captures CO2 best at cool temperatures, so they must figure out how to get rid of the heat that's produced quickly, so the CO2 binds. Another problem is exactly how to apply the compound. Can it be packed into smoke stacks? Can the material be produced into removable discs that cover smoke stack openings?

Although HAS may never be found in tailpipes, if the Georgia Tech researchers can lower carbon dioxide emissions from energy production alone, they will have offered one new way to solve our greenhouse gas troubles.

For more information on climate change and other related topics, visit the next page.

Wheeler, David and Ummel, Kevin. "Calculating CARMA: Global estimation of CO2 emissions from the power sector - working paper 145." Center for Global Development. May 2008. http://www.cgdev.org/content/publications/detail/16101/